Systematic study of disorder induced by neutron irradiation in MgB2 thin films

Abstract

The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 10 20 cm −2 . All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8 K. Residual resistivity progressively increases from 1 to 190 ⍀ cm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields ͑28 and 45 T͒ allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high ͑of the order of 50 T in the parallel direction at low fluences͒ and turns out to be rather independent of the experimental resistivity, at least at low fluences. A simple model to explain this phenomenology is presented.

Effects of Neutron Irradiation on Magnesium Diboride Thin Films

“…From the results of the fit, we calculate the scattering times in the two bands and thus the resistivity of the films. It results to be in good agreement with the experimental resistivity values [18], confirming the reliability of the analysis. Starting from the scattering times in and bands, we calculate the mean free paths , and compare them with the coherence lengths estimated from the gap values measured on irradiated bulks [20].…”

“…This irradiation experiment can be easily compared with ours, considering that the energy of the particles is very similar; in our case, we just have to take into account the presence of the Li nuclei, which is almost ten times more effective in damaging the crystal. If we scale our fluences taking into account this factor and the probability of a neutron capture by the , the critical temperatures and resistivities curves of the two experiments overlap quite well [18].…”

“…In the following, they will be referred to as IRR10-IRR50. In neutron irradiation, the damage is caused by the neutron capture reaction of atoms, which decay in high energy Li nuclei and particles, as widely described in [11], [18]. Despite the huge cross section of this reaction, the damage is quite homogeneous in thin films, whose thickness is lower than the neutron penetration depth (few hundreds of ).…”

Roles of intrinsic anisotropy and π-band pairbreaking effects on critical currents in tilted-c-axisMgB2films probed by magneto-optical and transport measurements

“…(8) obtained under the assumption of t >> ξ π for the films with t ≅ ξ π may require a phenomenological replacement of t with t + t 0 in the denominators to cut off the divergence at t = 0 so that J c does not exceed J d in thin films if t 0 ≅ ξ π ≅ 50 nm. We can also expect reduction of A due to suppression of superconductivity at the film surface due to lattice mismatch between the film and the substrate and oxides at the surface [45]. Moreover, the change of the nanostructure of surface steps for different film thicknesses shown in Fig.…”

Two-band parallel conductivity at terahertz frequencies in the superconducting state ofMgB2

“…20 This effect can be mainly attributed to an increase of ␥ , as shown by a detailed analysis of magnetoresistance data 19 and by recent firstprinciple computations. 24 In a reasonable approximation, the R S / R N ratio can be evaluated within the generalized BCS model by using the same parameters of the ultraclean film case, with the exception of ␥ increased from 11 to 37 meV.…”